[Beefy]

Hi Everyone,

I've built my 2.4 x 1.5m plasma table using dual chain drive on the X-axis. Did my very first test cut and then had a haunting thought about the accuracy implications of dual chain (or belt) drives which are driven from a single cross shaft as mine are.

I squared the gantry to the table at one end then realised that due to the high likelyhood of the chains not being equal lengths over a 2.4m distance, the gantry could be ran to the opposite end and would be out of square. And there's nothing that can be done about this. Even if the total length of chain was exactly the same length from new (for a given number of links of course) it would be unlikely that wear and stretch in the chain would occur equally and so you could only square the gantry to the table at one end but when at the opposite end would be out of square by the length difference of the chain.

So if anyone is considering a single shaft drive with only one drive motor and accuracy is a concern then maybe it would be better to spend the extra cash and drive each chain individually so the steps per mm / inch could be calibrated for each side.

After the above thoughts I think rack and pinion is the very best way for a plasma table. No stretch, more dirt resistant if they are mounted with the spur gear underneath, no tensioning issues.

Just a thought for anyone who's thinking of doing the same design as me. It's my first table and like a lot of other people I've been doing the money saving thing and have done a lot of living and learning as a result.

Anybody had any such issues with accuracy ?

All the best for the New Year.

Keith. :cheers:

[ger21]

So if anyone is considering a single shaft drive with only one drive motor and accuracy is a concern then maybe it would be better to spend the extra cash and drive each chain individually so the steps per mm / inch could be calibrated for each side.

If you use mach3, and slave two motors, they must be set exactly the same, so you can't calibrate them independently.

[Beefy]

Now that is something I didn't know. Thanks very much for that, it changes things quite a bit. I have been through the Mach manual several times yet I'm learning I can easily miss things. Now that you've told me that I think I can recall the manual saying the slaved axis recieves exactly the same pulses.

Seems then that the ONLY way to get the accuracy in question is to use dual gear racks where the manufacture has been accurate enough to give them "identical" lengths.

I'm actually still in a bit of shock over this new information. When money permits if didn't go the gear rack route, I would have invested in a 2nd motor and reduction pulleys, + another Gecko drive, for the second chain, yet have been in exactly the same position as I am now.

Well it's great to know where I stand now. Thanks again, your reply has educated me and saved me some future dummy spitting.

Keith

[ger21]

Just to be sure I'm giving you 100% correct info, I'll do some testing later today, with the steps per inch of the slave set to half that of the master, to see what it does. If it even lets me.

[jimcolt]

The industrial machines (read expensive) typically use a servo drive on each side of the gantry (usually inertia matched, ac brushless) coupled to a zero to low backlash planetary gear box and helical rack and pinion drive. Each side is treated as its own axis, and most machines leave the factory after being calibrated with a laser interferometer to compensate for gear rack tolerances. The Industrial CNC controls can map tolerances along the length of the motion, store them in memory, and compensate for them continuously. This technology may not be necessary on smaller footprint machines....but many industrial cnc machines have very long travel lengths.....I have seen 350' long machines in shipyards!

On the lower end of the spectrum, there are machines with a single gantry drive motor with a cross shaft and standard rack gears on both sides. These maintain accuracy and squareness pretty well. I have never seen a "chain drive" commercial cnc plasma....but there are a few that use long "toothed belt" drives....typically these are lower accuracy machines use for sheet metal HVAC (ductwork) applications.

Jim

[Beefy]

Gerry this is interesting. The Mach manual does indeed say that the slaved axis will recieve the same pulses as the master axis (during normal use it adds !), YET I can go into motor tuning and set different values for steps per mm/in. However I don't have dual drive axis to test this out. The very fact that the manual states "during normal use" seems to imply that this is not set in stone. If you manage to do those tests that will be very interesting. If not I'll do a separate post on this subject.

Jim, you've answered some questions that have been in my mind several times in the past. I sometimes suffer from thinking deeper than is necessary and one of those things was where a drive system could be worn at a point along its length, say where lots of cutting was done. The designers of those big machines obviously leave no stone unturned when it comes to setting up and maintaining accuracy.
I haven't noticed many people talking about how they set their machines up accurately, especially getting the gantry perpendicular to the X-axis. Apart from having a giant and accurate set square, or the method I used where I attach a fine tip pen and use Pythagoras to measure and see if I've got 90 degrees, have you heard of any other tricks (I guess the laser setup gear may be expensive to hire).

Cheers,

Keith.

[ger21]

I just went out and checked it. My X axis is 6400 steps/inch. I set the A axis to 3200 steps/inch. I jogged it back and forth, and it was no different
Mach3 appears to ignore the A axis setting, and the machine works the same as it does with both set the same.

[Beefy]

Thanks for that Gerry, guess I'm not getting off the hook that easily. The only way around this that I can think of at the moment would be to not have the A-axis slaved, and modify the post in my cam program so whenever a command is given to the x-axis, and identical command is automatically given to the A-axis. However, I have absolutely no idea how to do that.

Keith.

[Torchhead]

If you use mach3, and slave two motors, they must be set exactly the same, so you can't calibrate them independently.

Ger: You sure? My first servo plasma table when I calibrated each side was a little different on the steps per unit side to side. I was able to set the slave slightly different and it corrected the issue. I tested it over several feet of run and multiple times. Maybe I was dreaming......It's been several years now and that was a ballscrew table (bad mistake for plasma).

TOM C

[ger21]

Read my post #7 above.

[start here]

Hi Everyone,

I've built my 2.4 x 1.5m plasma table using dual chain drive on the X-axis. Did my very first test cut and then had a haunting thought about the accuracy implications of dual chain (or belt) drives which are driven from a single cross shaft as mine are.

I squared the gantry to the table at one end then realised that due to the high likelyhood of the chains not being equal lengths over a 2.4m distance, the gantry could be ran to the opposite end and would be out of square. And there's nothing that can be done about this. Even if the total length of chain was exactly the same length from new (for a given number of links of course) it would be unlikely that wear and stretch in the chain would occur equally and so you could only square the gantry to the table at one end but when at the opposite end would be out of square by the length difference of the chain.

So if anyone is considering a single shaft drive with only one drive motor and accuracy is a concern then maybe it would be better to spend the extra cash and drive each chain individually so the steps per mm / inch could be calibrated for each side.

After the above thoughts I think rack and pinion is the very best way for a plasma table. No stretch, more dirt resistant if they are mounted with the spur gear underneath, no tensioning issues.

Just a thought for anyone who's thinking of doing the same design as me. It's my first table and like a lot of other people I've been doing the money saving thing and have done a lot of living and learning as a result.

Anybody had any such issues with accuracy ?

All the best for the New Year.

Keith. :cheers:

I too am building a table using dual chain drive on the X-axis (but mine uses one motor per side) so I read your post with interest.

After a little thought of what you described, IMHO, I doubt it will be a problem. I think the chains or belts will need to operate under extreme tension for a long time before any significant stretch is measurable.
And if (or when) you find that it is starting to affect the accuracy of the cutting you should be able to simply tighten one side....
I would think a simple test can quickly determine if you have a problem. By temporarily disconnecting one side, get the table to move the gantry a known distance - say 2 meters, & mark that point. Now reconnect that side, disconnect the other side, and repeat the same 2 meter move. If the marks don't line up, you can see which one is further along and how much to tighten the chain on the opposite side.

[Beefy]

Hi Start Here,

Thanks for your input. Terry Parker has been using chain for years and he says it's never been an issue for him, but he told me something very important about chain drive and which way to do it, so I thought I should pass this onto you seeing as you are building one.

My table has the drive sprocket mounted at each side of the table, and idler sprockets at the opposite ends for tensioning. So the sprocket is stationary and the chain moves. Problem with this is you get a lot more chain "bouncing". Terrys system has stationary chain stretched the length of the table and his drive sprockets are mounted each side of the gantry. So in effect the chain is a rack and the sprockets are the pinions just like a rack and pinion setup. Even with my chains quite tight you still get the bounce especially on sharp turns and this fast bouncing shows up in the cuts. In my cam program I'll use "loop sharp corners" to try and prevent this, plus I'll use the smallest tips I can to keep the speed down. Not a great machine design.

The one thing that has interested me with a lot of the replies on this topic is that so many are saying they don't think it will be a problem. Nobody seems to have actually tested / measured their systems to verify it definitely is not a problem. I will be checking it out after all the other tasks I have on my plate but I personally could not just hope for the best and think it won't be a problem. Let me give you an example.

I did all the calculations to enter steps per mm in Mach. Synchronous pulley ratios, chain pitch and number of sprocket teeth per revolution. If the chain is manufactured so precisely then my steps per mm should have been dead on. I used Machs calibration feature to measure the distance travelled over the 2.4m run and it was not exactly as calculated. Tolerances are a very standard thing in any manufactured item so I didn't expect my steps per mm to match exactly. With my chain pitch there's 260 links over 2.4 metres. Each chain links allowable tolerance can add or subtract to a length when you've got 260 of them. If they do end up precisely the same length I would feel very lucky, and likewise I would be very surprised to find them stretching by exactly the same length. Like I say I will be doing initial tests on this and checking these lengths every so many hours of use.

Another thing is google some chain manufacturers and see what they have to say about initial wear-in stretch. Mine was predicted to be 2.4 mm longer after wearing in. Definitely not the type of thing I will think / hope will be OK.

At the end of the day it probably won't be an issue unless I'm cutting a large item where the sides need to be accurately square. Plasma cutting itself is not that accurate due to the bevel on the sides, and getting the kerf offset very accurate. The biggest issue I'm finding now is the flop in the chains affecting the cut quality, mainly after a sharp turn. When money and time allow I'll be swapping to rack and pinion with spring loaded pinions to give zero backlash. No stretch, no bounce.

Cheers,

Keith.

[jimcolt]

Keith,

Your quote about plasma cutting accuracy is 1. not correct and 2. Not really the issue when considering chain drive cnc plasma machines.

"Plasma cutting itself is not that accurate due to the bevel on the sides"

Todays best air plasma systems can cut with rather nice precision and excellent edge squarenes on most materials....in fact it is not uncommon for part tolerances to be held to .015". And that is air plasma when talking about industrial quality high definition class plasma...then edge squarenes, cut part accuracies, hole quality and fine features rival the best of laser cutting systems at about 1/3 the purchase price and 1/4 the operating cost.

No plasma system will produce excellent cut quality by itself....it needs excellent motion control (low following error), excellent acceleration and de-acceleration, the ability to maintain the recommended plasma profiling speed even on intricate parts of a cut, and it also requires excellent torch to work distance control for both piercing and for steady state cutting.

95% of the complaints I get regarding edge angularity, hole roundness and cut part accuracy are solved by improving part programming or machine motion issue! Mechanical backlash is one of the worst problems, with sluggish acceleration following pretty close behind.

While I see no problem with building a chain drive system, please don't place the tolerance burden on the process tool! The best cutting plasma, laser, water jet system is a well choreographed dance between the cutting process, the x and y motion control, the torch height control, the part program as well as the critical timing on each components funtionality....machine code!

Jim Colt Hypertherm

[start here]

Thank you for passing that info on Keith

I know what you mean about tolerances, and the repetitive errors that can occur when there are many links put together. I can live with an expected 2.4mm stretch as long as it doesn't keep stretching, and I don't think it will. I hope you update this thread with the results of any tests you may carry out. It would be a benefit to many of us.

My design is like Terry's, with the chain stretched out and idler pulleys on the gantry. I think I can get it tight enough to be as accurate as I need. Chain has no backlash when tensioned, and if it stretches (after the initial stretch), I can tighten it easy enough. Maybe it might be possible to get a very strong spring, and use that to maintain constant tension - don't know, it's an idea I might try.

But I'm keeping in mind that I don't expect the same level of cutting as when I get parts cut at the laser cutter. I think no matter how I drive it - chain, belt, R&P, etc, I will never get the same results. Mine is a home made plasma cutter table, and if it cuts straight without jaggered lines, it's good enough for me. Having said that, I certainly want to get it working as good as possible.....

If you plan to change to R&P, obviously you're not happy with the chain drive. Do you have any pics that might show the cut quality?

[Beefy]

Jim thanks for the correction to my statement. I guess I was referring to us DIY guys with our "low end" air plasma cutters (mine's a Powermax 1250 ) and our basic (relatively speaking) methods of machine control. I must remember to be more specific and not to make statements that sound very broad and appear to apply to all plasma systems.

Start Here, I tried to do a close up with my camera but I don't know how to get it in focus at that distance so I'll just have to try and describe it. It basically looks like triangular / sawtooth edge with the "teeth" about 1 mm apart. This was my very first cut just to check my machine worked. I have heard that some people are using Finecut nozzles on their Hypertherms up to 6 mm steel, my steel was 5mm and I was using 40 amp nozzles. I also was cutting the wrong direction so the left side of the plasma jet was on the part side, the right side of the jet gives less bevel. I reckon I'll get a dramatically improved cut if I use Finecuts and the much lower speed, and I loop sharp corners. I've got good acceleration on my table but the gantry jarring direction changes make the chain bounce quite fast despite the tension.

I don't know what torch height control you're using but Jim (Colt) mentioned in a different post that his plasmacam machine has a system where it automatically sets the voltage at the correct cut height. That is is measures the cutting voltage at the start of the cut while the torch is the recommended height above the material, then after that the voltage height control takes over. This helps to adjust for things like the hafnium electrode wear causing a greater arc length (= higher voltage). My system is from Candcnc and it has recently had this type of feature added, it's called Smart-Kut. I never quite understood what the big deal was at first. As a beginner I was always focussed on controlling arc volts via adjusting the cutting height. Now I realise that it's the other way around, the voltage is an indication of the cut height (that's the important bit) so Jims Plasmacam and Candcnc Smart-Kut feature automatically set the voltage at the correct cut height.
It does need an accurate initial height sensing setup to work though. Just thought I'd let you know about that one if you don't know about them already.

Jim, let me know if I got anything wrong in the above.

Cheers,

Keith.

[start here]

So as long as you're cutting 25TPI hacksaw blades, there's no problems

Thanx for the tip re Smart-cut, I wasn't aware of it. Just went to Candcnc and did some reading. As I'm trying to build mine on a budget (shoestring), I was going to go with his LCTHC but haven't purchased anything yet. Will need to do more research to see if it's worth spending the extra $$

I see you're downunder, where abouts? I'm in Melb

[Torchhead]

There is a comparison chart at:
http://www.CandCNC.com/DTHCCompareChart.htm
that shows the two products and what features each one has. The LCTHC is a basic Digital Torch Height. It does have the Tip Saver (anti-dive) feature but lacks any integration with MACH past the standard inputs. Here are a couple of things to consider: The MP3000-DTHC is also a full Breakout and interface with expanded Inputs and outputs. (gets two ports of I/O out of one port). You will need some form of BoB, AND you will probably need a 2nd parallel port for the THC inputs since conventional BoB's only have 4 open inputs (after e-stop) for everything. The THC takes 3.

If you total up the cost of the added components you are going to need to get motion, homes/limits and THC signals plus the LCTHC you may find the price differential is minimal

You may not think all the bells and whistles of the DTHC are needed (depends on how you defined NEED) but anything that helps you get better cuts and easier setup will come in welcome later down the learning curve.

Personally, I think the on-screen readout out of the Cut Current using the DCP-01 is about the neatest thing I have done lately but HEY, I'm the guy that designed it so it's my baby no matter how ugly it is

TOM caudle
www.CandCNC.com

[Beefy]

Start Here,

before I forget I'm in Melbourne too, Frankston area.

Tom took the words out of my mouth. Before I seen his post at the bottom I was reading yours about trying to save money. I reckon I have saved nothing by buying all the parts and putting everything together. The biggest thing is how much time goes in to learning, sourcing, etc.

I have learnt a lot along the way but next time I'd get a complete package from Tom. You won't be the first one I've advised to go with a Candcnc package. Time savings, less headaches, quality components and probably an equivalent price. The built in features for plasma cutting are great (my opinion YOU NEED DIGITAL TORCH HEIGHT CONTROL if you want cut quality, or don't want your nozzle to crash into thin metal when it warps with the heat, it's not an option. Also the cost will be recouped in time from longer lasting consumables). From what I've seen, his packages are an expression of his passion to create a full featured plasma table control system for the likes of you and me. He seems to be adding software and hardware features all the time. Think hard before you go the "money saving" route like I did. Mustn't forget to mention very good support from Tom too, check out the Yahoo support groups.

Sheetcam is relatively cheap and has a post processor for Toms system. Very capable and easy to use, plus very good support from its creator Les, another guy who seems to add features as time goes by. It's a system a lot of people are using together with Candcnc packages.

Cheers,

Keith.

[start here]

Tom & Keith, I really appreciate the advice. Now all I have to do is take it

Having spent some time on the site, one thing is for sure, there is a lot to take in. To be honest, I'm not sure I follow what you're saying Tom, re. expanded Inputs and outputs, BOB (second BOB?), etc, but I am a member of your Yahoo group and read the emails, learning as I go. You've certainly done a great job designing/manufacturing those products and I've seen your dedicated support through your Yahoo group. If I already didn't have my motors & drivers, I'd be buying a package from you.

Keith, hindsight is wonderful, isn't it? I know what you mean about trying to save $$ but going about it the wrong way. False economy, those small amounts spent add up quickly, before you know it you've spent twice what you thought you would. Add to that the time you spent and come to the realization that in the end you've saved little but ended up with something no where near as good.....happens to me occasionally. Hopefully not this time

In my case, I'm not building a "dream table". To be honest, I just wanted to see what can be done with minimal outlay. You may have seen my thread "Plasma table - do-able for $500 -$1K"
I'll probably make a few mistakes as I go, most of us usually do when we make something for the first time. I'm OK with that. That's how I (we?) learn. Too late to turn back, I have almost everything required now - steppers, drivers, PS, drive mechanisms, steel, cases, wiring, and it looks like I will complete the build well and truly within my budget. Initially it won't have any THC but will have a z axis so I can add one later on. I think that there are some things that you need to try out for yourself. Plus it can be "fun" for some people but frustrating for others.....
Hey, the thing could end up crap and may have been a total waste of time & money, or it might turn out really well and I won't hesitate to spend the $$ for the MP3000-DTHC. I may even get hooked on this CNC thing (like I'm not already) and use the table to build "The One", which without doubt, will have Tom's full PlazPak-1 package on it. I'll know soon enough....